Indra Nooyi's exit is a reminder of how much needs to be done to boost women CEO ranks

The number of women leading some of America's largest companies, already tiny, shrunk even more Monday.

PepsiCo CEO Indra Nooyi, an iconic leader, announced she would step down in October and Pepsi President Ramon Laguarta would replace her. With the departures of Nooyi, Mattel's Margo Georgiadis and Campbell Soup's Denise Morrison this year, women will lead just 24 out of the 500 companies in the S&P 500.

That means women represent less than 5 percent of chief executives in the group.

"The reality is that we have more work to do if we want to see progress for women in the very top level of companies," said Heather Foust-Cummings, senior vice president of research at Catalyst, a nonprofit that promotes women in the workplace.

Nooyi became PepsiCo's first female chief executive in 2006 and pushed the company toward a more nutritious portfolio and a more diverse business in her 12-year tenure. She's spoken about the challenges women leaders face, including how they're held to a different standard than men, as well as the need to find a solution for women balancing corporate careers and families.

Women make up more than half of the workforce, according to Department of Labor statistics, but fewer make it to the top of the corporate ladder. Some leave their jobs to care for children or aging parents. Others who stay in the corporate world tend to lose ground compared to their male peers, according to a report from McKinsey & Company and nonprofit organization Lean In.

Their latest study found women hold only 21 percent of senior vice president roles and "since the vast majority of CEOs come from line roles, this dramatically hurts women's odds of reaching the very top."

"I think in any organization, people are more comfortable with people with similar backgrounds, perspectives," said Karen Kosiba Edwards, a partner at Boyden executive search firm. "Those are the easiest people to choose, mentor and promote. But what's easy is not necessarily going to make you a leader or a winner in the kinds of challenges that we're all facing today. I think we've all gotta stretch ourselves a little bit."

In addition to Pepsi's Nooyi and Campbell's Morrison, the consumer products industry lost Mondelez CEO Irene Rosenfeld late last year. Campbell has not yet picked Morrison's successor, though one likely candidate is Luca Mignini. If he or another man is chosen, that will mean each of the three females will be replaced by a man.

"Progress isn't linear," Shannon Schuyler, PwC's corporate responsibility leader and a spokeswoman for the CEO Action for Diversity & Inclusion organization, said in a statement. "That said, we need to address the complex mix of issues that contributes to the lack of advancement for underrepresented groups, which includes increasing the talent pipeline, helping boards prioritize diversity in succession planning, as well as acknowledging how the intersection of race and gender can impact progression into management."

Boyden'sEdwards said companies are trying to hire diverse people in both the C-Suite and corporate boards. Some shareholders are even demanding it.

BlackRock, the largest money manager in the world with over $6.28 trillion in assets under management, earlier this year said companies it invests in should have at least two female board members. State Street last year said it would push companies to hire more diverse board members.

So far, it appears progress is being made. Women accounted for 31 percent of new board directors at 3,000 of the largest publicly traded U.S. companies, according to a data analysis by corporate governance firm Institutional Shareholder Services.

However, Catalyst's Foust-Cummings said companies need to do more than simply hire more women. Instead, they need to invest in improving the corporate culture.

"If you simply change the numbers but do nothing to change the culture, you're unlikely to see numerical changes to the company over time," she said.